Fiber Optics Increase Sensor Range

Photonics.comOct 2004
BLACKSBURG, Va., Oct. 22 -- Researchers at Virginia Tech University's Center for Photonics Technology say they are on their way to solving a problem that is limiting the range and number of sensors used to safeguard civil and industrial infrastructures.

Real-time monitoring of oil and gas fields and pipelines, power networks, bridges, dams and buildings, for instance, is important to economic and homeland security. Using sensors with different capabilities to create large, economical sensor networks has been limited by the distance a signal will travel and by a limited multiplexing capability. Sensor multiplexing in a fiber sensor system means the sensors along a fiber cable can be interrogated by one optoelectronic signal-processing unit. The optical signal is first converted by light detection into an analog signal, which is then digitized for further signal processing.

Researchers at the Center for Photonics Technology (CPT) have discovered new methods for fabricating and spacing sensors within optic fibers, called ultraviolet-induced intrinsic Fabry-Perot interferometers (IFPIs), and have demonstrated that the resulting fiber optic sensors have a greater range.

"We believe we could place many more sensor elements along a single fiber cable, and these fiber cables, placed in different geographical areas, could be linked by a computer network," said Anbo Wang, professor of electrical engineering at Virginia Tech and director of the CPT. "In theory, many such computer 'networks' can be linked to form a nationwide system."

Now Wang and his colleagues have received a $500,000 Sensor Initiative Research Grant from the National Science Foundation to develop "Highly Multiplexed Optical Fiber Sensing Networks for Infrastructure Monitoring."

Infrastructure monitoring requires sensors that can cover a large area with minimum maintenance, ultralow cost per unit and the ability to operate in harsh environments of different kinds. Semiconductor-based electronic sensors are low-cost and can use wireless transmission to cover a large area. However, they are susceptible to electromagnetic interference (EMI) and restricted to relatively low temperatures (most below 125 C). One solution is to combine electronic sensors with the more expensive optical fiber sensors, which are insensitive to EMI, offer great resolution and accuracy and have much higher temperature capability. A roadblock has been the rather limited multiplexing capability, usually within several hundred sensors along a single fiber cable.

Wang's group is confident that their sensor technology will increase the multiplexing capability by at least one order of magnitude.

"In addition, this capability can be multiplied by many fold through sensor data fusion and computer networking, so millions of sensors of different types in one system may become possible for real-time key infrastructure monitoring," said Wang.

Fabry-Perot interferometers have been used for temperature, strain, pressure, electromagnetic field nd ultrasound sensing. The Virginia Tech experiments demonstrated that the new UV-induced IFPI fiber sensor can also measure different physical parameters, including temperature, strain and pressure, and that they can operate at temperatures above 600? C.

Sensor performance characteristics will be optimized through continued work on sensor fabrication and multiplexing using both mathematical modeling and experimental analysis.

The research program will also educate a multidisciplinary workforce for sensor and network development through the support of postdoctoral associates and graduate, undergraduate and high-school students in the areas of electrical engineering, materials science, mechanical engineering, mathematics and computer engineering.

In addition to Wang, the researchers are Gary Pickrell, assistant professor of material science and engineering; Kristie Cooper, research scientist in electrical engineering; Luiz DaSilva, associate professor of electrical and computer engineering; and Tao Lin, professor of mathematics.

Any device in which variations in the transmitted power or the rate of transmission of light in optical fiber are the means of measurement or control. Fibers can be used to measure temperature, pressure, strain, voltage, current, liquid level, rotation and particle velocity.